1. Field of the Invention
The invention relates to proppants useful in oil and gas wells. In particular, it relates to a ceramic proppant material in which proppants of various sizes are included, and will be described with particular reference thereto. It will be appreciated, however, that the invention is also suited to the extraction of other fluids from boreholes, such as water wells.
2. Discussion of the Art
Oil and natural gas are produced from wells having porous and permeable subterranean formations. The porosity of the formation permits the formation to store oil and gas, and the permeability of the formation permits the oil or gas fluid to move through the formation. Sometimes the permeability of the formation holding the gas or oil is insufficient for economic recovery of oil and gas. In other cases, during operation of the well, the permeability of the formation drops to such an extent that further recovery becomes uneconomical. In such circumstances, it is common to fracture the formation and prop the fracture in an open condition by means of a proppant material or propping agent. Such fracturing is usually accomplished by hydraulic pressure using a gel-like fluid. The pressure is increased until cracks form in the underground rock. The proppants, which are suspended in this pressurized fluid, are forced into the cracks or fissures. When the hydraulic pressure is reduced, the proppant material functions to prevent the formed fractures from closing again.
A wide variety of proppant materials are used, depending on the geological conditions. Typically, proppants are particulate materials, such as sand, glass beads, or ceramic pellets, which create a porous structure. The oil or gas is able to flow through the interstices between the particles to collection regions, from which it is pumped to the surface. Over time, the pressure of the surrounding rock tends to crush the proppants. The resulting fines from this disintegration tend to migrate and plug the interstitial flow passages in the propped structure. These migratory fines drastically reduce the permeability, lowering the conductivity of the oil or gas. Conductivity is a measure of the ease with which oil or gas can flow through the proppant structure and is important to the productivity of a well. When the conductivity drops below a certain level, the fracturing process is repeated or the well is abandoned.
Ceramic proppants, sometimes called man-made proppants, are favored over natural proppants, such as sand or resin-coated sand, due to their ability to withstand high pressures and temperatures and their resistance to corrosion. Despite being of higher cost than natural materials, the increased crush strength of ceramic renders the ceramic proppants suitable for conditions which are too severe for other materials, e.g., at rock pressures of above about 350 to 700 kg/cm2(5000–10,000 psi). As pressure increases with depth, ceramic proppants are commonly used at depths of about 1500 meters, or more. They are typically formed by combining finely ground material, such as clay, bauxite, or alumina, with water and then mixing in a rotary mixer. Blades in the mixer cause the wet clay to ball up into generally spherical pellets, which upon drying and firing at high temperature are of the general particle size desired. Pellets which fall outside the desired range are returned to the mixer after the drying stage to be reworked.
The crush strength of the proppants is related to the composition and density of the ceramic. Proppants are generally classed in one of three grades: light weight proppants (LWP), intermediate grade proppants (IP), and high strength proppants (HSP). Light weight proppants are suitable for use over a range of closure stress from less than about 1000 psi to about 8000 psi, while intermediate grade proppants are useful up to about 10,000 psi, and high strength proppants can be used at pressures in excess of 12,000 psi. Attempts to improve conductivity have focused on methods of improving crush strength of the proppants. These include the application of coatings, production of stronger spheres, and changes in shape. While measurable improvements in conductivity have been obtained, for example, by applying a resin coating, such improvements have invariably been associated with increases in cost.
Spherical pellets of uniform size have conventionally been considered to be the most effective proppants as they have been thought to maximize conductivity (see, e.g., U.S. Pat. No. 4,623,630). An excess of fines (very small pellets) acts to clog the void space in between the packed spheres, reducing the fluid transport. It is also known that spheres tend to be weaker as the size increases, and are thus more likely to become crushed in situ. In addition to increases in the number of fines, crushing results in a reduction in the width of the crack formed in the fracturing process. Thus, the presence of both small and large particles in a proppant mixture has been thought to be deleterious. Accordingly, the American Petroleum Industry (API) standard, the commonly accepted standard in the industry, requires that the particle size distribution be within fairly narrowly defined limits. For example, particle size ranges are defined according to mesh size designations, such as 40/70, 30/50, 20/40, 16/30, 16/20, and 12/18. The first number in the designation refers to the ASTM U.S. Standard mesh size of the largest (top) sieve and the second number refers to the mesh size of the smallest (bottom) sieve. The API standards require that 90% of the spheres comprising the proppant material be retained between the top and bottom sieve when sieved through the mesh designations for the product. A proppant material sold by Saint-Gobain Ceramics & Plastics, Inc., Stow, Ohio, under the tradename Versaprop™ has a broader distribution range than the API standards for a 20/40 material require. However, this material is not suited to all geological features, due to its small size.
As a result of the requirements for narrow particle size distributions, only a small proportion of the pellets produced in the forming process are within the predetermined range. The remainder, often as much as 75–80% of the material, must be reground or otherwise treated and reformed in the rotary mixer.
The present invention provides a new and improved proppant material and method of making and use which overcome the above-referenced problems and others.